1. Field of the invention
The present invention relates to a signal distinction circuit for detecting a discrimination signal having a predetermined frequency, and more particularly, to a signal distinction circuit capable of preventing a malfunction in distinguishing the discrimination signal due to, for example, a multi-path interference.
2. Description of the related art
An FMX stereophonic broadcasting is proposed to improve the S/N of an FM stereophonic broadcasting and enlarge a service area. According to the FMX stereophonic broadcasting, a compressed stereophonic difference signal is transmitted simultaneously with the transmission signal of the FM stereophonic broadcasting. The compressed stereophonic difference signal modulate a subcarrier of 38 KHz in quardrature with respect to a subcarrier of 38 KHz which is AM-modulated by a stereophonic difference signal. A discrimination signal of 10 Hz is included in the transmission signal of the FMX stereophonic broadcasting so that the FMX stereophonic broadcasting is distinguished from the FM stereophonic broadcasting. Therefore, in order to receive the FMX stereophonic broadcasting, it is necessary to detect the discrimination signal and switch the receiving mode of a receiver to the FMX stereophonic broadcasting receiving mode.
The FMX stereophonic broadcasting is described in detail on pages from 11 through 15 of a magazine "JAS JOURNAL" for September, 1986.
The discrimination signal of 10 Hz can be detected by a signal distinction circuit as shown in FIG. 3. Referring to FIG. 3, a stereophonic pilot signal of 19 KHz applied to a first input terminal 1 is divided into approximately 74 Hz by a first frequency divider 2 and then, applied to a counter 3 as a clock signal. The discrimination signal of 10 Hz applied to a second input terminal 4 is applied to a pulse generator 6 through a bandpass filter 5. Therefore, a pulse signal having the same frequency as that of the discrimination signal applied to the input terminal 4 is generated by the pulse generator 6. The pulse signal thus produced is divided by a second frequency divider 7, and then, supplied to the counter 3 as an actuating signal. Consequently, the counter 3 counts the pulse number of the clock signal while the level of the pulse signal is maintained to be an "H" level. A detector 8 detects whether or not a value counted by the counter 3 equals to a predetermined value.
However, the signal distinction circuit shown in FIG. 3 generates a signal indicating that it is receiving the transmission signal of the FM stereophonic broadcasting when the discrimination signal of 10 Hz is subjected to an interference such as a multipath interference and thus the average frequency thereof is changed. As such, although the receiver is receiving the signal of an FMX stereophonic broadcasting, the receiver is switched to receive the signal of the FM stereophonic broadcasting. In particular, the interference such as the multipath interference continues for a short period of time. Therefore, when the interference is terminated, the receiver is switched to receive the FMX stereophonic broadcasting. Thus, the receiver is unnecessarily switched two times.
In addition, when the receiver is switched to listen to a broadcasting which transmits a signal including a discrimination signal, the discrimination signal is distorted and consequently, its level is lowered when the discrimination signal is subjected to the multipath interference or when an external noise is mixed therewith. It is desirable that the receiving mode of the receiver is not changed as long as the listener desires to keep listening to the above-described broadcasting. The following problem occurs according to the known receiver. That is, for example, if the level of the discrimination signal of 10 Hz instantaneously falls while an FMX stereophonic broadcasting is being received, the receiver is switched to receive an FM stereophonic broadcasting. As a result, a noise level fluctuates, which causes a listener to feel uncomfortable.